Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a tower and a rotor arranged on the tower. The rotor, which typically comprises a hub and a plurality of blades, is set into rotation under the influence of the wind on the blades. The rotation may be transmitted (optionally through a rotor shaft) to a generator, either directly or through the use of a gearbox. This way, the generator produces electricity which can be supplied to the electrical grid.
During operation of a wind turbine, its structure and components may undergo undesired movements, i.e. oscillatory or repeating displacements in any direction (fore-aft vibrations, side-to-side or lateral vibrations, longitudinal vibrations, torsional vibrations, . . . ) of any magnitude and of any frequency (high or low, constant or varying). If a wind turbine is subjected to the above mentioned collateral movements (e.g. vibrations) during a prolonged period of time, fatigue damage may result.
These undesired movements may occur more easily at specific rotor speeds (rpms) that, depending on the particular configuration of the wind turbine, may induce structural resonance phenomena. In most circumstances, due to the inherent wind variability, modern variable speed wind turbines do not operate at such potentially dangerous rpms for a prolonged period of time. As the wind changes, the rpms may be varied.
Wind turbines are often grouped together in so-called wind farms. A wind farm control is generally aimed at centrally managing and/or controlling the wind farm such that the wind farm as a whole generates, usually according to a planned power generation, a total power to be supplied to the grid. This centralized control of the wind farm may generate suitable individual commands or set-points for the individual control systems of each wind turbine to cause generation of a corresponding amount of active power. The total power generated by the wind farm then should substantially correspond to the active power planned for the wind farm as a whole.
In case of unplanned grid unbalances, a grid operator may issue one or more online requests for the wind farm to vary its power production for compensating said unbalances. Most grid unbalances may refer to an excess of power in the grid, in which case an online request may be generated by the grid operator for the wind farm to reduce the power it is generating.
These online “corrections” may cause at least some of the wind turbines to generate less power than they could, which in most cases is achieved by reducing the rotational speed (rpms) of the rotor. Thus, wind turbines may be ordered to work in a range in which they may experience some undesired movements (as the ones explained before) and corresponding fatigue (in the long term). Additionally, it may be necessary to maintain a wind turbine's operation in such a regime for a prolonged period of time. A wind variation may not necessarily influence the wind turbine's performance to a large extent as it is already producing less than it potentially could.
The present disclosure aims at improving wind farm control and in particular at improving wind farm control in situations wherein a grid requirement is received.